Coordinate positioning apparatus

ABSTRACT

A three-coordinate measuring machine has a probe (22) connected to an output member (11). A driving system (10) is made up of three carriages (15, 17, 19) on guide rails (14, 16, 18). In parallel with this, there is provided a driven system (20) made up of three carriages (25, 27, 29) on guide rails (24, 26, 28). The carriages 19, 29) are connected together via the output member (11). The other pairs of carriages (15, 25 and (17, 27) are linked via respective devices (55). The devices (55) each apply a force between the respective carriages in dependence on acceleration or deceleration of the carriages of the driving system (10). This compensates for dynamic deflections of the guide rails of the driven system (20) which would otherwise be caused by the accelerations. Measuring devices such as scales and readheads are provided on the driven system (20) to determine the position of the probe (22).

BACKGROUND OF THE INVENTION

This invention relates to coordinate positioning apparatus. A typicalapplication of the invention is in three-dimensional measuring machinesi.e. in coordinate positioning apparatus adapted for determining thespatial measurement, in three dimensions, of workpieces or otherobjects.

In such machines, the taking of a measurement involves moving a toolsuch as a surface-sensing probe into sensing relationship, e.g. intophysical engagement, with a surface the position of which is to bedetermined. Readings of the position in space of the movable parts ofthe machine relative to the fixed parts are then taken, e.g. frommeasuring devices such as scales. There are machines where the probe ismoved automatically, at desirably high speeds, from one surface to thenext of a workpiece to be measured. At each surface the probe has to bedecelerated when approaching the surface and accelerated when beingwithdrawn from the surface to be moved to the next surface. The economyof measuring complex workpieces depends on the speed at which themachine can be operated.

However, in conventional measuring machines the movable components,especially the movable bridges found in such machines, are relativelymassive because of the need for stability of measurement. Less massivecomponents would tend to be less stiff, and would suffer dynamicdeflections when accelerated and decelerated. Such components requirelarge forces for acceleration and heavy foundations to provide reaction.Therefore, such machines are intrinsically not suitable for high speedoperation; there is a conflict between speed and stability ofmeasurement.

SUMMARY OF THE INVENTION

One aspect of the present invention provides coordinate positioningapparatus for positioning a tool comprising:

an output member connected to said tool;

a driving system, including at least one first, drivable carriage; guidemeans for guiding said first carriage in a first direction; and a firstelongate element connected to said output member and extending from thefirst carriage in a second direction substantially perpendicular to thefirst direction;

a driven system, including at least one second, driven carriage; guidemeans for guiding said second carriage in the first direction; and asecond elongate element connected to said output member and extendingfrom the second carriage in the second direction;

said tool being subject to positional inaccuracy caused by bendingdeflection of a said elongate element upon acceleration or decelerationof a said carriage;

the apparatus further comprising means for applying a force between thefirst and second carriages, in dependence on said acceleration ordeceleration, in a sense to reduce said positional inaccuracy.

A second aspect of the present invention provides coordinate positioningapparatus for positioning a tool, comprising:

an output member connected to said tool;

a driving system, including at least one first, drivable carriage; guidemeans for guiding said first carriage in a first direction; and a firstelongate element connected to said output member and extending from thefirst carriage in a second direction substantially perpendicular to thefirst direction;

a driven system, including at least one second, driven carriage; guidemeans for guiding said second carriage in the first direction; and asecond elongate element connected to said output member and extendingfrom the second carriage in the second direction; and

closed loop means having driving signal means for driving the firstcarriage; measuring means for measuring the displacement of the secondcarriage thereby produced; and feedback signal means for feeding thedisplacement thus measured back to the driving signal means.

As an example of the present invention, a coordinate measuring machinewill now be described with reference to the accompanying drawingswherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly sectioned front elevation of the machine showing adriving and a driven system in operational relationship.

FIG. 2 is a plan view of FIG. 1.

FIG. 3 is an enlarged section on the line III--III in FIG. 1 and showsadditional detail.

FIG. 4 is a partial and enlarged view of FIG. 2 and shows additionaldetail.

FIG. 5 is a perspective view of parts of the machine shown in FIGS. 1and 2 but, for clarity, the driving and driven systems are shown inexploded relationship.

FIG. 6 shows part of a modified machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring primarily to FIGS. 1, 2 and 5, a driving coordinate system 10supports an output member 11 for three-dimensional displacement relativeto a base 30. The member 11 is connected by a universal joint 12 (FIG.5) to an input member 21 of a driven coordinate system 20 which supportsa contact-sensing probe 22 for three dimensional displacement relativeto the base 30. The coordinate directions of this displacement aredenoted X, Y and Z.

In use of the machine, the driving system 10 drives the probe into asensing relationship with the surface of a workpiece placed on the base30. The probe then triggers the taking of X,Y,Z coordinate readings frommeasuring devices, which as described later are located on the drivensystem 20.

The driving system 10 has a pair of frame members 13 supported at threepoints 31,33,35 (FIG. 2) on a frame 13A upstanding from a ground surface37. The members 13 have secured thereto a pair of horizontal tracks 14each supporting a carriage 15. A pair of horizontal tracks 16, securedto the carriages 15 by arms 15B, support a pair of carriages 17 to whichare secured a pair of vertical tracks 18 supporting a carriage 19 towhich the output member 11 is secured. The tracks 14,16,18 ar linear andare mutually perpendicular thereby providing support for saidthree-dimensional displacement of the member 11. The carriages 15,17,19are driven along their respective tracks by respective motors15A,17A,19A (FIG. 4), controlled by a computer 38 programmed to drivethe motors to position the output member 11 in accordance withrespective drive signals 39 of a closed control loop 40. Since theindividual movements of the carriages 15,17,19 along their respectivetracks are component movements which are combined to produce themovement of the member 11, the system 10 may be referred to as acomponent combiner.

The driven system 20 has a pair of frame members 23 supported at threepoints 32,34,36 (FIG. 2) on the frame 23A supported on the frame 13A anditself supporting the base 30 (FIG. 1). The members 23 have securedthereto a pair of horizontal tracks 24 each supporting a carriage 25. Apair of horizontal tracks 26, secured to the carriages 25 by arms 25B,support a pair of carriages 27 to which are secured a pair of verticaltracks 28 supporting a carriage 29 to which the input member 21 issecured. The tracks 24,26,28 are linear and are mutually perpendicular,and the system 20 constitutes a three-dimensional component resolver forany displacement of the member 21, since it resolves any suchdisplacement into an X-component (along tracks 24), a Y-component (alongtracks 26) and a Z-component (along tracks 28).

The system 20 includes three opto-electronic measuring devices, fromwhich the X,Y,Z coordinate readings are taken by the computer 38 whentriggered by the probe 22. The measuring devices include measuringheads, 25A,27A,29A (FIG. 4) for measuring the displacement of therespective carriages 25,27,29 along their respective tracks. The tracks24 and 26 are provided with scales 24A,26A which are read by therespective measuring heads 25A,27A for this purpose, and the track 28similarly has a scale (not shown) which is read by the measuring head29A. Other types of measuring devices can be used if desired. Eachmeasuring head has a respective output signal 41 connected to thecomputer 38 and forming part of the closed control loop 40 to providefeedback for positioning the respective motors 15A,17A,19A. In otherwords, whereas the system 10 is driven by the drive signals 39, feedbackis provided by the signals 41 from the system 20. The drive signals 39thus provide error signals of the closed control loop 40.

It will be clear that there is at least one track 14 of the system 10parallel and proximate to a corresponding track 24 of the system 20 andthe two systems have two corresponding carriages 15,25. The samedescription applies to the tracks 16,26 and the carriages 17,27 as wellas to the tracks 18,28 and the carriages 19,29. This arrangement makesit possible for both systems 10,20 to be constructed of relatively lightcomponents essentially because deflection of the system 10 is not sensedby the measuring devices which, as mentioned, are provided on the system20. For the purpose of control stability, measuring devicescorresponding to the devices 25A,27A,29A may also be provided in thesystem 10 to provide a "secondary" position feedback. The system 10 mayalso include a velocity control loop in respect of the movement of thecarriages 15,17,19.

The potential for light construction is exploited especially well by theintroduction of compensating devices now to be described.

The systems 10,20 are connected by two gravity-compensating devices 45(FIGS. 3,4) intended to support the tracks 24,26 against the deflectionunder gravitational force. Further, the systems 10,20 are connected bytwo inertia-compensating devices 55 intended to provide compensation inrespect of dynamic deflection of the arms 25B and the tracks 26,28 dueto inertia in the system 20 when a force is applied thereto at the inputmember 21. The devices 45,55 will be described in detail.

FIG. 4 shows the two devices 45 denoted respectively 45X,45Y. The device45Y is shown more specifically in FIG. 3 and comprises a compressionspring 46, connected between an adjuster 47 provided on the carriage 17and one end of a lever 48 connected by a pivot 48A to the carriage 17,the other end of the lever being provided with a roller bearing 49 onthe underside of the adjacent carriage 27. The arrangement is such thatthe spring 46 supports the tracks 26 against deflection under its weightand the weight of the carriage 27, the tracks 28, the carriage 29, themember 21 and the probe 22. As a result, the forces which would causedeflection of the track 26 are reacted in the track 16 and the track 26remains linear at the expense of deflection in the track 16. FIG. 3shows the device 45 as provided between the corresponding carriages17,27 at the top end of the tracks 18,28. An equivalent device 45 may beprovided between the corresponding carriages 17,27 at the bottom end ofthe tracks 18,28.

The device 45X (FIG. 4) is provided between the carriages 15,25 and iscontructed in the same way as the device 45Y. It will be understood thatthere is one device 45 between each pair of the corresponding carriages15,25.

Further FIG. 4 shows the two devices 55, denoted respectively 55X,55Y.The device 55Y comprises a mass 56 secured to one end of a double-endedlever 57 connected to the carriage 17 by a pivot 58. The other end ofthe lever 57 is connected to the carriage 27 by a pivot link 59. It willbe clear that any acceleration of the carriage 17 in the direction ofthe track 16 has to act through the tracks 18,28 and any consequentdeflection of the tracks 18,28 results in the carriage 27 lagging behindthe carriage 17 until acceleration ceases. The device 55Y compensatesfor such lagging inasmuch as the inertia of the mass 56, acting throughthe lever 57 and the link 59, applies to the carriage 27 a force in thedirection of the acceleration of the carriage 17 so that the carriage 27substantially moves together with the carriage 17 notwithstandingdeflection of the tracks 18,28. The mass of the mass 56 and the lengthof the lever 57 are selected (relative to the stiffness and lengths ofthe tracks 18,28 and the masses of the system components) to achievethis. The mass 56 effectively comprises a means for sensing accelerationcombined with a means for generating a force proportional to theacceleration sensed. The force has to be reacted by the arms 15B andtracks 18. Deflection of the latter arms and tracks under said reactionreduces the force, but the effect of this is negligible provided thatthe stiffness of the arms 15B and tracks 18 is reasonably high comparedto the masses to be accelerated and given that the carriages aresupported on their tracks by virtually frictionless air bearings.

A second said device 55, denoted 55X, is similar to the device 55Y butis arranged to act between the carriages 15,25 in the direction of thetracks 14 so that acceleration of the carriage 25 in the latterdirection is compensated for in respect of deflection of the tracks16,18,26,28 in that direction.

In as much as the devices 55 are intended to remove inertia forces onthe system 20, these devices may also be regarded as intended toeliminate dynamic loads between the carriages and their respectivetracks transverse to the length of the track, caused by acceleration anddeceleration of the systems. So, for example, when the carriage 15 isaccelerated in the X-direction, the force then acting on the joint 12 inthe X-direction causes transverse loads on the carriages 29 and 27. Inas much as these forces are due to the inertia of the carriage 25, theacceleration of the carriage 25 by the device 55X eliminates thesetransverse forces from the carriages 27,29. However, consideration maybe given to the other inertial masses and to the compliance of any ofthe members of the system 20. Firstly, there are the arms 25B each ofwhich is both an inertial mass and, to some extent at least, a compliantmember. Both these aspects can be dealt with by providing, instead ofthe device 55X, two such devices (not shown) each connected between thearms 15B,25B in positions 15X2 (FIG. 5) respectively above and below thecarriage 25. More specifically, said two devices have positionsintermediate between the carriage 25 and the respective tracks 26 soselected that, while the arm itself can bend, the relative position ofthe carriage 25 and the ends of the tracks 16 remain unchanged.

Further, consideration may be given to deflection of the tracks 16 inthe X-direction due to inertia of the carriages 27,29 and theirassociated tracks 26,28 in that direction. The latter inertia can becompensated for by a device 55X3 (FIG. 4) provided between the carriagesof each carriage pair 17,27 but arranged to act in the X-direction sothat inertia forces acting in that direction are equalized or, in otherwords, transverse loads on the carriages 27 are eliminated.

In a modification illustrated in FIG. 6, and referring by way of exampleto the carriages 15,25, the device 55 is replaced by a device 65comprising an accelerometer 60 mounted on the driving carriage 15, and alinear motor 62 provided between the carriages 15,25. The linear motorcomprises a coil 64 secured to one of the carriages 15,25 and a core 66secured to the other one of these carriages. The accelerometer 60 isadapted to produce an output signal proportional to acceleration of thedriving carriage 15. An amplifier 64 is provided for amplifying saidoutput signal and the output of the amplifier is connected to the linearmotor 62, the latter being adapted to produce between the carriages15,25 a force proportional to said acceleration.

The accelerometer 60 could be mounted on the driven carriage 25 insteadof the driving carriage 15. Indeed, this may well be preferable: we wishto reduce or eliminate dynamic deflections of the driven system in whichthe measuring devices are provided, and the accelerations of the drivencarriage are more directly related to these. It will be clear that suchaccelerometer and linear motor devices can be provided between thecarriages 17,27 or wherever appropriate between the driving and drivensystems 10,20.

The invention is not restricted to bridge-type coordinate measuringmachines such as exemplified above, but can be used in any machine inwhich a probe or other tool is to be positioned at a given coordinatelocation. For example, it can be used in a coordinate measuring machineof the type in which a probe is mounted on a cantilever arm. In thiscase there would be a driving system and a driven system each comprisinga cantilever arm.

We claim:
 1. Coordinate positioning apparatus for positioning a tool,comprising:an output member connected to said tool; a driving system,including at least one first, drivable carriage; guide means for guidingsaid first carriage in a first direction; and a first elongate elementconnected to said output member and extending from the first carriage ina second direction substantially perpendicular to the first direction; adriven system, including at least one second, driven carriage; guidemeans for guiding said second carriage in the first direction; and asecond elongate element connected to said output member and extendingfrom the second carriage in the second direction; said tool beingsubject to positional inaccuracy caused by bending deflection of one ofsaid elongate elements upon acceleration or deceleration of one of saidcarriages; the apparatus further comprising accelerationresponsive meansfor applying a force between the driving system and the driven system,in dependence on said acceleration or deceleration, to reduce saidpositional inaccuracy.
 2. Coordinate positioning apparatus according toclaim 1,wherein the driving system includes a further, drivablecarriage, guided for movement generally perpendicular to said first,drivable carriage, and forms a component combiner for combining mutuallyperpendicular displacements of the drivable carriages into resultantdisplacement of the output member, and wherein the driven systemincludes a further driven carriage, guided for movement generallyperpendicular to said second, driven carriage, and forms a componentresolver for resolving the displacement of the output member intorespective displacements of the driven carriages.
 3. Coordinatepositioning apparatus according to claim 2, wherein the first elongateelement forms a guide means for said movement of the further drivablecarriage, and wherein the second elongate element forms a guide meansfor said movement of the further driven carriage.
 4. Coordinatepositioning apparatus according to claim 2 wherein saidacceleration-responsive means comprises at least twoacceleration-responsive devices for applying a force in dependence onacceleration or deceleration, one of said devices acting between thefirst, drivable carriage and the second, driven carriage, and another ofsaid devices acting between the further drivable carriage and thefurther driven carriage.
 5. Coordinate positioning apparatus accordingto claim 1, wherein said acceleration-responsive means for applying aforce in dependence on acceleration or deceleration comprises a leverpivoted between the driving system and the driven system, the leverbeing provided with a mass the inertia of which provides the force independence on acceleration or deceleration.
 6. Coordinate positioningapparatus according to claim 1, wherein said acceleration-responsivemeans for applying a force in dependnece on acceleration or decelerationcomprises an accelerometer mounted to measure said acceleration ordeceleration, and a linear motor provided between the driving system andthe driven system, the accelerometer having an output signal whichcontrols the linear motor to produce said force in dependence onacceleration or deceleration.
 7. Coordinate positioning apparatusaccording to claim 1, including gravity-compensating means forsupporting at least one of said guide means against deflection undergravitational force.
 8. Coordinate positioning apparatus according toclaim 1, including measuring means for measuring the displacement of thesecond carriage.
 9. Coordinate positioning apparatus for positioning atool, comprising:an output member connected to said tool; a drivingsystem, including at least one first, drivable carriage; guide means forguiding said first carriage in a first direction; and a first elongateelement connected to said output member and extending from the firstcarriage in a second direction substantially perpendicular to the firstdirection; a driven system, including at least one second, drivencarriage separate from said first carriage; guide means for guiding saidsecond carriage in the first direction; and a second elongate elementconnected to said output member and extending from the second carriagein the second direction; and closed loop means having driving signalmeans for driving the first carriage along its guide means in said firstdirection; measuring means for measuring the displacement of the secondcarriage thereby produced; and feedback signal means for feeding thedisplacement thus measured back to the driving signal means. 10.Coordinate positioning apparatus according to claim 9,wherein thedriving system includes a further, drivable carriage, guided formovement generally perpendicular to said first drivable carriage, andforms a component combiner for combining mutually perpendiculardisplacements of the drivable carriages into resultant displacement ofthe output member, and wherein the driven system includes a further,driven carriage guided for movement generally perpendicular to saidsecond, driven carriage, and forms a component resolver for resolvingthe displacement of the output member into respective displacements ofthe driven carriages.
 11. Coordinate positioning apparatus according toclaim 9,said tool being subject to positional inaccuracy caused bybending deflection of one of said elongate elements upon acceleration ordeceleration of one of said carriages; the apparatus further comprisingaccelerationresponsive means for applying a force between the drivingsystem and the driven system, in dependence on said acceleration ordeceleration, to reduce said positional inaccuracy.
 12. Coordinatepositioning apparatus for positioning a tool comprisinga componentcombiner adapted to combine respective displacement of a first and asecond driving member into resultant displacement of an output member; acomponent resolver having an input member connected to said outputmember and adapted to resolve the displacement of said output memberinto respective displacements of a first and a second driven member;said tool being mounted on one of said output and input members; firstclosed loop means having driving signal means adapted to drive saidfirst and second driving member and feedback signal means identifyingthe displacement of said first and second driven member respectively.13. Apparatus according to claim 12 comprising means for determiningacceleration of at least one of said driving members, and meansconnected between said one of the driving members and an adjacentcorresponding one of said driven members for applying to the drivenmember a force being a function of said acceleration.
 14. Coordinatepositioning apparatus for positioning a tool, comprising;a firstpositioning system, including at least a first movable carriage; guidemeans for guiding said first movable carriage in a first direction; anda first elongate element connected between said tool and said firstmovable carriage, the first elongate element extending in a seconddirection generally perpendicular to the first direction and beingsubject to bending deflection upon acceleration or deceleration of saidfirst movable carriage, such deflection tending to cause positionalinaccuracy of said tool; and acceleration-responsive means for applyinga force to the first positioning system, in dependence on saidacceleration or deceleration, said force acting to reduce saidpositional inaccuracy.
 15. Coordinate positioning apparatus according toclaim 14, wherein the first positioning system includes a furthermovable carriage, guided for movement in the second direction by saidfirst elongate element, said tool being connected to said first elongateelement via said further carriage, and wherein theacceleration-responsive means is connected to said further carriage sothat said force is applied to said further carriage.
 16. Coordinatepositioning apparatus according to claim 14, further comprising a secondpositioning system, including a second elongate element which extendsgenerally in the second direction, generally parallel to the firstelongate element, and which is guided for movement in the firstdirection, and wherein said acceleration-responsive means applies saidforce between the first positioning system and the second positioningsystem.
 17. Coordinate positioning apparatus according to claim 16,wherein the first and second positioning systems each include arespective further movable carriage, said further carriages being guidedfor movement in the second direction by the first and second elongateelements respectively, said tool being connected to at least one of saidfurther carriages, and wherein the acceleration-responsive means isconnected between said further carriages so that said force is appliedbetween said further carriages.